Quote Originally Posted by scottmcl
FP4+ in Acutol 1:15. Water stop. Double fix in Sprint Fixer. Wash. 2nd bath was fresh fixer. Wash (tap) using Patterson thingy. Sprint Fixer remover. Wash some more. Finally, distilled water + alcohol + some FotoFlo. Hang to dry in a clothes hanging bag (keeps dust off).

So no powdered chemicals. I first imagined that somehow the silver was clumping of something during fixer, but the FofoFlow is an intriguing theory.

Really odd, yes?

Yes!.. it is the silver emultion clumping during fixing. Why? depents on film emultion and how old the fixer is (common in 5x4 film plates from Eastern Europe).
Solution? Clean the film gently with your fingers inside a wetting agent solution. Use a clear tray on top of a light sourse if you can, only if it is safe!!!

All the best.